It is preferred to use planar antennas to radiate a preferred wave mode. A planar antenna which can be used in connection with the device according to the invention is described in the book entitled "Einfuhrung in die Theorie und Technik planarer Mikrowellenantennen in Mikrostreifenleitungstechnik" ("Introduction to the Theory and Technology of Planar Microwave Antennas in Microstrip Line Technology"), Gregor Gronau, Verlagsbuchhandlung Nellissen-Wolff or in the journal article "Impedance of a Radiating Slot in the Ground Plane of a Microstrip Line", IEEE Trans. Antennas Propagat., vol. AP-30, 922-926, May 1982.
A planar antenna comprises, for example, a dielectric substrate on one side of which the antenna structure is provided, and on the other side of which a conductive coating is provided. Openings are arranged in the conductive coating in such a way that the antenna radiates only electromagnetic waves of the desired mode.
Special protective measures are to be taken when the filling level meter is used in an area subject to explosion hazards. Here, it is imperative to ensure that overvoltages cannot be discharged through the explosive medium onto a neighboring surface at ground potential. Thus, in particular, no discharge spark may be allowed to jump over from the antenna onto the flange with the aid of which the filling level meter is fastened on the container. Overvoltages on the antenna are caused, for example, by a lightning strike.
A known protective measure provides that before it strikes the filling level meter an overvoltage is intercepted by an additional device, a so-called lightning protection system. This solution is, of course, relatively expensive. It has also become known to implement overvoltage protection in an electronic way.